An example gas discharge lamp is the type commonly referred to as a fluorescent lamp. This is generally a mercury vapour plus inert or rare gas (argon or a mixture of similar gases) at low pressure, sealed into an elongated glass tube that is provided with electrodes at each end. As the name suggests, visible light is emitted from fluorescing phosphors coating the inside of the glass tube when excited by ultraviolet light. A fluorescent lamp is normally driven from the AC mains (110 V, 60 Hz or 240 V, 50 Hz etc) and must be used in series with a ballast (to limit current; the lamp having a negative "resistance" when in its running mode) and must be used with starting gear, to heat the electrodes during startup and to provide a high-voltage starting pulse. (The term "resistance" here applies to a voltage, temperature, and history-dependent property (are ions present?) which is rather an impure resistance). This ancillary circuitry results in a high capital cost for a fluorescent lamp installation and the starter device is at least as likely to fail during use as is the lamp itself.
Furthermore, it is difficult to provide brightness control of a fluorescent lamp. Yet an easily dimmable lamp would be useful in controlled illumination arrangements, in "intelligent buildings" and in lecture theatres, for example.
Another example gas discharge lamp is widely known as the neon tube as used in a neon sign. These devices comprise long and usually specially bent glass tubes terminated with electrodes and containing various rare gas fillings (including but not limited to the gas neon) at low pressure, optionally made of coloured glass and sometimes also with coating phosphors hence providing a wide variety of colours.
Typically each lamp is driven at mains frequency (50/60 Hz) by a potentially lethal 15 kV, 30 mA current limiting transformer. This voltage is used to ionise the gas filling and start the lamps. Once started, these lamps run at a lower voltage and, being gas-filled, also exhibit a negative resistance.
Furthermore, brightness control of a neon sign is very difficult to achieve. AC power control at the primary of the transformer is the usual method, resulting in on/off control only.
An isolated lamp power supply has likely safety benefits. Applications for the operation of gas discharge lamps under adverse (wet) conditions include lighting for environments where salty water exists--such as on board ship, in wharves, fish processing plants, meat processing plants, farm dairies, hotel kitchens, in coal or other mines, and the like.
Another "wet" application is in illumination of spaces or in illuminated signs used outdoors, where contact by the public with a wet sign, perhaps in a case which has deteriorated, is likely to result in lethal shock. Condensation in a window of a restaurant for example is likely whenever the weather outside is cold, or humid.
A further "wet" application is a swimming pool.
A yet further "wet" application is in the sterilisation of liquids--actually using biocidal wavelengths and substantial doses of ultraviolet light in order to sterilise liquids that may contain micro-organisms harmful to life. The working conditions under which lamps of this type are used would benefit from the use of simple, reliable, isolated lamp power supplies.
Yet another application is in testing products for resistance to ultraviolet degradation where existing accelerated test methods prescribe a certain type, positioning, and number of lamps but at present cannot prescribe that they emit a controlled amount of light.